✨(oidc) add django-oauth-toolkit w/ configuration

This allows to use `people` as an identity provider using
OIDC and local users.
This commit is partial, because it does not manage a way to
create "local" users and the login page is the admin one, which
can't be used for non staff users or login with email.

src/backend/mailbox_manager/utils/dimail.py

@@ -8,6 +8,7 @@ from email.headerregistry import Address
 from logging import getLogger
 
 from django.conf import settings
+from django.contrib.auth.hashers import make_password
 from django.contrib.sites.models import Site
 from django.core import exceptions, mail
 from django.template.loader import render_to_string
@@ -341,6 +342,7 @@ class DimailAPIClient:
                             # secondary email is mandatory. Unfortunately, dimail doesn't
                             # store any. We temporarily give current email as secondary email.
                             status=enums.MailboxStatusChoices.ENABLED,
+                            password=make_password(None),  # unusable password
                         )
                         imported_mailboxes.append(str(mailbox))
                     else: